Here are the details in the words of Terry@BMS:
Got back from a couple hours of dyno tuning and came back with some interesting results. This car definitely likes E85...
The JB4 now has full electronic wastegate control in addition to the fuel mass control & CANbus, paving the way for E85 testing. The test car is our 2015 DCT M3, 100% stock with a JB4, with ethanol mixed with 91 octane at 30-50%.
I've been running E30 for the last week on a tune that ramps up to around 25psi and I've been very impressed with it. On the dyno today running that same trim resulting in a very consistent ~510whp / ~510wtq results. Although the turbo holds boost to redline it looks like shifting around 6000 rpm will result in the best acceleration results as power noses off above that.
While I had it strapped down I decided to explore more aggressive power levels. Added a little more E85 and went on an hour long adventure through many different curves and tweaks. Finally arrived at a 28psi peak tune which yielded ~540whp / ~575wtq. I've not tested this much on the street so we'll have to see how viable it is. But this just goes to show you how much room was left on the table for tuning with this platform.
This car is an animal.